1. Field of the Invention
The present invention relates to a high voltage power supply device for lighting a discharge tube, and more particularly, to a high voltage power supply devices for lighting discharge tubes having a fault protection circuit such as inverter power sources for liquid crystal panel back lights in portable information devices.
2. Description of the Related Art
FIG. 7 is a circuit diagram showing an example of a cold cathode tube lighting inverter using a conventional fault protection circuit. In FIG. 7, the cold cathode tube lighting inverter contains an inverter unit 10 and a fault protection circuit 20. To light a cold cathode tube 2, the inverter unit 10 comprises a step-up transformer 1, a tube current control circuit 3, a drive circuit 4, and a resistor 5 as a current-voltage converter, and a rectifier circuit 9. The drive circuit 4 produces an AC signal for driving the step-up transformer 1, corresponding to an input voltage, and feeds the signal to the step-up transformer 1. The step-up transformer 1 increases the voltage of the AC signal, and provides the signal to one of the electrodes of the cold cathode tube 2 to light the cold cathode tube 2.
The resistor 5 is connected between the other electrode of the cold cathode tube 2 and ground. A tube current flowing into the resistor 5 causes a voltage. The voltage is rectified in a rectifier circuit 9 comprising a diode 6, a resistor 7, and a capacitor 8. The rectification voltage Vrct is fed to the tube current control circuit 3. The tube current control circuit 3 controls the drive circuit 4 so that the rectification voltage Vrct becomes substantially equal to a desired constant value. In this way, the tube current is controlled to be substantially constant, due to the operation of the respective parts of the inverter unit 10. As a result, the brightness (brilliance) is also controlled to be substantially constant.
The fault protection circuit 20 comprises a resistor 21, a transistor 22, a capacitor 23, a constant-current source 24, and a thyristor 25. A remote signal is provided to the On-Off terminal of the tube current control circuit 3 via a resistor 26, and also, to the anode of the thyristor 25. The cathode of the thyristor is grounded. The rectification voltage Vrct output from the rectifier circuit 9 is given to the base of the transistor 22 via the resistor 21. The emitter of the transistor 22 is grounded. The gate terminal of the thyristor 25 and the constant-current source 24 are connected to the collector. The fault protection capacitor 23 is connected between the collector of the transistor 22 and ground.
Next, operation of the cold cathode tube lighting inverter shown in FIG. 7 will be described. When the cold cathode tube 2 lights normally, the tube current flows through the resistor 5, and the rectification voltage Vrct is thereby fed to the base of the transistor 22 via the resistor 21 of the fault protection circuit 20. Thus, the transistor 22 is able to conduct, and a charging current, caused by the constant-current source 24, bypasses the fault protection capacitor 23. Thus, no voltage is stored in the fault protection capacitor 23. As a result, the voltage at the gate terminal of the thyristor 25 is not increased, so that the thyristor 25 remains off, and the On-Off terminal of the inverter unit 10, maintained at the H level, continues to operate normally.
If the cold cathode tube 2 is not connected or malfunctions, no tube current flows in the resistor 5. Thus, the rectification voltage Vrct of the rectifier circuit 9 becomes zero, and the transistor 22 becomes unable to conduct. Thereby, a charging current from the constant-current source 24 flows into the fault protection capacitor 23. The gate voltage of the thyristor 25 is increased by a time constant value determined by the amount of the existing charging current and the electrostatic capacitance of the fault protection capacitor 23. When the gate voltage exceeds the constant value, the thyristor 25 is turned on, the on-off terminal of the inverter unit 10 reaches the L level, and the operation of the inverter unit 10 is stopped. That is, the circuit configuration is such that protection is provided if the cold cathode tube 2 is not connected or malfunctions.
FIG. 8 is a circuit diagram showing another example of a conventional cold cathode tube lighting inverter.
In FIG. 8, an inverter unit 30 includes a tube current control circuit 33 in addition to the step-up transformer 1 and the drive circuit 4 shown in FIG. 7. In this example, characteristically, the tube current control circuit 33 is AC-coupled by capacitor 34 to the cold cathode tube 2. The other electrode of the cold cathode tube 2 is grounded via a resistor 35. One terminal of a capacitor 34 is connected to the node of the cathode and resistor 35. The other terminal of the capacitor 34 is connected to the base of a transistor 38. The base of the transistor 38 is the input terminal of the tube current control circuit 33. A constant-current source 36 and a diode 51 are connected in series with each other. The voltage at the node is fed as a bias voltage Vf to the input terminal via a resistor 37.
This bias voltage Vf is cancelled out by the base-emitter voltage Vbe of the transistor 38. If at that time, the diode 51 and the transistor 38 are in the same chip, the temperature characteristic of the bias voltage Vf and that of the base-emitter voltage Vbe can be completely cancelled out. That is, it is assumed that the capacitor 34, the constant current source 36, the diode 51, the resistor 37, and the transistor 38 constitute an ideal diode which eliminates the bias voltage Vf. Then, the peak voltage of the voltage Vfb obtained by voltage-conversion of the tube current and the peak voltage of the rectification voltage Vrct, which is the emitter voltage of the transistor 38, are equal to each other.
A resistor 39 and a capacitor 40 are connected in parallel to each other between the emitter of the transistor 38 and ground. The rectification voltage Vrct is fed to the comparison input terminal of a comparator 41. A target voltage Vcnt is applied to the standard input terminal of the comparator 41. The rectification voltage Vrct is compared with the target voltage Vcnt by the comparator 41. The output is fed to an integration circuit 51 and integrated therein, and is input to an on-duty modulation circuit 42. The on-duty modulation circuit 42 controls the on-duty of the drive circuit 4 so that the average rectification voltage Vrct and the target voltage Vcnt become equal to each other. Thus, the tube current of the cold cathode tube 2, and moreover, the brilliance of the cold cathode tube 2 are controlled to have a constant value.
The conversion voltage Vfb, obtained by converting the tube current to a voltage by means of the resistor 35, is also input to the comparison input terminal of a comparator 43 in an fault protection circuit 50. A reference voltage Vudr is applied to the reference input terminal of the comparator 43. The output from the comparator 43 is fed to the base of a transistor 45 via a resistor 44. The emitter of the transistor 45 is grounded, and the collector is connected to the gate terminal of a thyristor 48. A constant-current source 47 is connected to the collector of the transistor 45, and a fault protection capacitor 46 is connected between the collector and ground. When the conversion voltage Vfb exceeds the reference voltage Vudr, the comparator 43 outputs an H level signal, causing the transistor 45 to be turned on, so that the charge stored in the fault protection capacitor 46 is discharged.
If the cold cathode tube 2 is broken, is not connected, or the like, resulting in no tube current, output from the comparator 43 is maintained at the L level. Thus, the voltage across both terminals of the fault protection capacitor 46 is increased by a time constant value which is determined by the constant current source 47 and the electrostatic capacitance of the fault protection capacitor 46. When the terminal voltage of the fault protection capacitor 46 reaches the on-voltage of the thyristor 48, the thyristor 48 is turned on, so that the operation of the inverter unit 30 is stopped.
In the above-described conventional example shown in FIG. 7, the tube current control circuit 3 is DC-connected to the other electrode of the cold cathode tube 2. Therefore, the accuracy of the tube current depends on the Vf of the diode 6. The Vf has a temperature characteristic. Thus, a problem arises in that when the ambient temperature changes, the tube current value of the cold cathode tube 2 changes.
The temperature characteristic of Vf is about 2.5 mV/xc2x0 C. For example, in the case of an inverter of which the specified temperature range is zero xc2x0 C. to 60xc2x0 C., the change in Vf is xc2x12.5 [mV/xc2x0 C.]xc2x760[xc2x0 C.]=150 mV. To reduce the effects of this Vf change, a voltage generated in the detection resistor 5 is increased.
For example, to reduce the tube current variation caused by this temperature change to 1% or smaller, it is required that the voltage generated in the detection resistor 5 is about 150 mV÷1%=15Vo-p (zero to peak)=10.6 Vrms or higher.
If a cold cathode tube for lighting a liquid crystal panel with a size of about 2 to 2.5 inches is selected as the cold cathode tube, the tube voltage will be about 200 Vrms. That is, the power loss caused by incorporation of the detection resistor 5 is large, that is, 10.6÷(200+10.6)=5%. Thus, a problem arises in that if the change in tube current which accompanies a change in ambient temperature is suppressed, the power loss increases due to the higher resistance value of resistor 5.
On the other hand, in the conventional example shown in FIG. 8, the tube current control circuit 33 is AC-coupled to the other electrode of the cold cathode tube 2. The voltage Vf of the diode 51 and the base-emitter voltage Vbe of the transistor 38 cancel out each other. Therefore, the dependency of the tube current on the ambient temperature is not caused in principle. Accordingly, it is not necessary to considerably increase the detection resistance 35. The power loss can be suppressed to be small compared to the example shown in FIG. 7.
However, in the case of formation of a one-chip IC using the AC-coupling configuration, Vcnt less than Vf is selected, as the base terminal of the transistor 38 avoids the application of negative voltages.
FIGS. 9A and 9B illustrate wave-form charts at a conversion voltage Vfb, a target voltage Vcnt, and a rectification voltage Vrct for large and small rectification time constants which are determined by the resistor 39 and the capacitor 40 shown in FIG. 8.
Since the tube current is controlled so that the average of the target voltage Vcnt and that of the rectification voltage Vrct are equal to each other, the areas of the oblique line portions A and B shown in FIG. 9A are equal. Thus, as could be understood, when the rectification time constant is small, the peak voltage of the conversion voltage Vfb can be controlled to be relatively high, compared to the target voltage Vcnt. However, with the rectification time constant being gradually increased, the peak voltage of the conversion voltage Vfb converges to the target voltage Vcnt.
When the rectification time constant is decreased, the dispersion of the tube current tends to increase, as a result of dispersions in constants of the resistor 39 and the capacitor 40, that is, dispersions in time constant. For this reason, it is preferable that the rectification time constant is increased as much as possible from the viewpoint of the tube current accuracy. Thus, in many cases of practical design, the peak voltage of the conversion voltage Vfb is set to be substantially equal to Vcnt.
As seen in the above description, the peak voltage of the conversion voltage Vfb becomes less than the bias voltage Vf. Thus, even if the conversion voltage Vfb is provided directly to the transistor 45 as shown in the conventional example of FIG. 8, the transistor 45 is not turned on. Therefore, a problem arises in that an expensive comparator 43 needs to be incorporated.
Since a delay in lighting of the cold cathode tube 2 is caused in some cases, it is required that the voltage be continuously output for one to several seconds, although no tube current flows directly after the start. Therefore, in both of the conventional examples of FIGS. 7 and 8, each of the time constants, determined by the constant current source and the fault protection capacitor capacitance, is set at one to several seconds.
In the described circuits, a problem arises in that if there is a fault, such as generation of arc discharge, which is caused by partial disconnection of a high voltage wire, e.g., the fault protection circuit may fail to operate. In the case of arc discharge, the flow of the discharge tube current may start and stop at intervals shorter than a time of second order. Thus, with the conventional protection circuit, stopping-operation is impossible. In the worst case scenario, the circuit may be damaged, due to abnormal heating.
Accordingly, it is an object of the present invention to provide a high voltage power supply device for lighting a discharge tube in which the temperature dependency of the tube current accuracy can be solved, in which the power loss caused by the current detection resistance can be reduced, and which will stop the high voltage supply for faults such as partial high voltage wire disconnection.
To solve the above problems, according to the present invention, there is provided a high voltage power supply device for lighting a discharge tube which comprises a current-voltage converter that detects a current flowing through a discharge tube, and converts the current to a voltage for output, a tube current controller having an input portion thereof AC-coupled to the current-voltage converter and which controls the tube current flowing through the discharge tube so that the current becomes substantially constant, a protection circuit that stores an electric charge in a fault protection capacitor, recognizes the occurrence of a fault corresponding to the voltage across both terminals of the fault protection capacitor exceeding a predetermined value, and stops the control by the tube current controller, an impedance element connected between the discharge tube and the current-voltage converter, and a reset circuit that prevents the fault protection capacitor from being charged so that the protection circuit is not operated while current flows through the discharge tube corresponding to the voltage generated in the impedance element.
Accordingly, the temperature-dependence of the tube current accuracy is eliminated, AC coupling is employed. Thus, the power loss, caused by the current detection resistance, can be reduced.
According to another aspect of the present invention, there is provided a high voltage power supply device for lighting a discharge tube which comprises a current-voltage converter that detects a current flowing through a discharge tube, and converts the current to a voltage for output, tube current converter having an input portion thereof AC-coupled to the current-voltage converter and which controls the tube current flowing through the discharge tube so that the current becomes substantially constant, a protection circuit that stores an electric charge in a fault protection capacitor, recognizes the occurrence of a fault corresponding to the voltage across both terminals of the fault protection capacitor exceeding a predetermined value, and stopping the control by the tube current controller, and a time constant circuit for discharging the fault protection capacitor during a time determined by a first time constant after starting, the protection circuit having a circuit for charging the fault protection capacitor by a second time constant value which is shorter than the first time constant, wherein, immediately after starting, fault protection is carried out by the first time constant value, and after a predetermined time from the starting, the fault protection is carried out by the second time constant value.
Accordingly, the voltage can be continuously output during a period when lighting of the discharge tube lags immediately after the starting, so that protection can be provided against defects such as disconnection of high voltage wiring.
Preferably, the second time constant is set at 10 milliseconds or shorter.
As a result, effective protection can be also realized against partial disconnection of a high voltage wiring.
Also, preferably, the high voltage power supply device for lighting a discharge tube further comprises dimming means for changing the lighting duty ratio of the discharge tube for burst dimming corresponding to a dimming signal, by use of the tube current controller, and a hold circuit for maintaining a voltage across both terminals of the fault protection capacitor during a burst off period by use of the dimming means.
Accordingly, satisfactory burst dimming and protection-operation can be realized.
Also provided in accordance with the invention is a fault protection circuit for a high voltage power supply for lighting a discharge tube.